A shock absorber of this type is known from European patent application 0,221,602.
The purpose of controlling the fluid flow in a shock absorber is to adapt the damping properties thereof as far as possible in an optimum manner to the momentary traveling conditions of a vehicle equipped with one or more of such shock absorbers. In motor vehicles it is for example important to keep the wheels in constant contact with the road as far as possible during moving off, acceleration, sudden sharp braking or when the motor vehicle leans to the side. It goes without saying that the type of road surface also plays an important role in the selection of the damping characteristics of the shock absorber. In general, if the road surface is good and fairly even, relatively little damping will be selected, while in the case of a poor, uneven road surface the shock absorbers have to be set with relatively great damping effect.
In the known shock absorber the sleeve is displaceable in axial direction of the central aperture by electrically controlled means, such as an electromagnet having a coil in which a core is movably arranged. The core is axially connected to the sleeve in its center. The sleeve has axial through flow channels, having cross-section dimensions such that the flow of damping fluid through the central aperture is not or in a neglectable manner impeded.
The damping characteristics of this know shock absorber are determined by the position of the sleeve in the central aperture, in other words, through which duct system(s) fluid can flow to the spaces on either side of the piston in the cylinder. With this arrangement the damping characteristics of the shock absorber can be controlled proportionally, independently of the speed of travel or direction of movement of the piston. Depending on the number of duct systems and their respective flow characteristics, the final damping characteristic of the shock absorber can be set proportionally over a wide range of speeds of travel of the piston.
The choice of the cross-section dimensions of the through flow channels in the sleeve or in general a sleeve-type element for the control of the fluid flow in the central aperture, is also based on the effort to make the response speed of the control as far as possible independent of the fluid pressure in the aperture.
As is well known, the response time of an electromagnet depends on the speed with which a magnetic force of sufficient strength can be built up to move the core. Generally, the response time of an electromagnet decreases if the magnet has to provide for an increased magnetic strength. However, for an effective control of the damping characteristics of shock absorbers there is a need for a response time of the order of few tenths of milliseconds or less. Such a response time cannot be achieved by a shock absorber having an electromagnet as the means for displacement of the sleeve.
From the European patent application 0,236,204 a shock absorber is known, having a piston with a first duct system which is shut off by means of spring-loaded valves which open or close under the influence of the speed of travel and the direction of movement of the piston, and a second duct system which is formed by a central aperture in the piston, and in which the fluid flow can be allowed through or held back by means of a displaceable body. Said body can be displaced through the activation of a wire memory metal.
Although a wire memory metal has the potential for achieving the intended response time, in the embodiment of the shock absorber disclosed by EP-A-0,236,204 the displaceable body is a pin-type element for opening or closing off the central aperture. With this shock absorber it is only possible to achieve a control system similar to an ON/OFF control, with which the damping characteristics of the shock absorber cannot by any means be adapted in the optimum manner to the mentioned traveling conditions. Because the fluid flow through the first duct system is determined only by the speed of travel and the direction of movement of the piston, allowing through or shutting of the fluid flow in the central aperture by means of the pin-type element will result in hardly any appreciable change in the damping characteristics upwards of a certain speed of travel of the piston.
Through activation of the element of memory metal the pin-type element is moved parallel to the fluid flow in the central aperture, for shutting it off. During the movement of the piston in the cylinder the fluid pressure causes a force opposed to the force exerted on the pin-type element by the wire of memory metal. Since a relatively narrow duct has to be opened or closed in the central aperture, the fluid pressure in this duct will rise relatively high, and the force exerted thereby on the pin-type element will consequently be relatively high, which has an adverse effect on the response time of the shock absorber control.